The invention is directed to a pressure-operated power wrench.
Power wrenches, as disclosed e.g. in U.S. Pat. No. 4,805,496 are provided with a headpiece having an annular member rotatably supported thereon. The annular member is engaged by a lever including a ratchet, which lever can be pivoted by the power of a hydraulic piston displaceably arranged in a cylinder. The unit consisting of the piston and the cylinder can be a single-acting unit wherein the piston is pushed into its retracted position by a spring, or a double-acting unit wherein the pressure chamber is provided on one side of the piston and a counterpressure chamber is provided on the other side, with the pressure chamber and the counterpressure chamber being alternately pressurized and depressurized.
Known hydraulic power wrenches suffer from the disadvantage that, in a working stroke, the whole pressure chamber on the one side of the piston has to be filled with pressure oil. Due to the hydraulic resistance of the duct feeding the pressure chamber and due to the valves contained in said duct, the pressurizing of the pressure chamber requires a relatively long time with each working stroke. In the initial phase of the rotating of a bolt, the load moment of the bolt is small, so that, for moving the piston, already a relatively small force would be sufficient. Nevertheless, during each working stroke, the whole pressure chamber is filled with pressure oil. This relatively large oil throughput, with a large quantity of oil being pressed through hoses and valves, further results in the oil being excessively heated. Thus, in the pressure aggregate, consisting of a compressor and a pressure container, there is required a correspondingly large cooling capacity whereby the pressure aggregate becomes expensive and bulky.
In the power wrench of U.S. Pat. No. 4,919,018, the drive means comprises, instead of the otherwise common sole cylinder, two parallel cylinders which are moved synchronously with each other and in common generate the power for moving the lever and for rotating the annular member. Using two cylinders with appertaining pistons has the purpose of multiplying the force of a single cylinder unit so as to effect larger screwing moments. Concerning oil throughput and the time required for rotating the bolt, however, the conditions are the same as in a power wrench having one cylinder only.